It is well known that by leading heated water into a solar tank through a stratification device from the bottom of the tank, good thermal stratification can be achieved in the tank. The thermal stratification is achieved because the stratification device ensures that the heated water in the stratification device only enters the tank when the temperature in the tank equals the temperature in the stratification device or when the water in the stratification device is forced to leave the stratification device at the top thereof. Good thermal stratification enhances the thermal performance of solar heating systems.
Different types of stratification devices are known. DE 102 12 688 A1 and DE 39 05 874 A1 (owned by the company Solvis GmbH) disclose a tank with a vertically arranged stratification device comprising a rigid inlet pipe having a number of outlet openings provided with flaps that work as non-return valves. This known stratification device is comparatively complicated and thus expensive. Further entering water can only enter the tank at the level of the openings which might not be at the right level according to the thermal conditions of the water in the tank. As a result, the stratification of the water in the tank might be impaired.
Furthermore, WO 80/01714 A1 discloses a device for use in a tank containing liquid and wherein the liquid is caused to attain a condition of equilibrium, in which the temperature of the liquid increases in a steady manner from the bottom to the surface, ie. is stratified.
The tank houses a unit comprising two chambers mounted one above the other. The upper chamber is supplied with heated liquid via a pipeline and the lower chamber is supplied with cooled liquid via a pipeline. The two chambers each have two side walls, ie, an inner wall and an outer wall, formed of two concentric, radially interspaced, perforated cylinders.
The cylinders are made from a material possessing low thermal capacity and poor thermal conductivity in order to have as slight effect as possible on the stratification in the tank. The perforations of the cylinders allow for liquid flow therethrough.
In order to reduce the risk of disturbing the stratification in the tank, with radial spacing the chambers are enclosed by an inner cylindrical filter fabric and an outer filter fabric. The outer filter fabric is distended by means of three annular plates. The inner filter fabric is held in position by means of the inner circumference of the annular plates.
Additionally, JP 11037682A discloses a heat storage tank provided with so-called diffusers arranged horizontally. Each diffuser comprises an inner tube and an outer tube. An axially extending opening is formed in the outer tube. A large number of holes oriented in directions approximately opposite to that of the opening of the outer tube are formed in the inner tube.
Moreover, stratification devices consisting of a single inlet pipe formed of a porous fabric are known. Examples of such stratification devices, which are also called fabric stratifiers or fabric stratification manifolds, are disclosed in the articles: “Fabric Stratification Manifolds for Solar Water Heating”, J. H. Davidson and D. A. Adams, Transaction of the ASME, vol. 116, August 1994, pages 130-136, and “Stratification Enhancement in Liquid Thermal Storage Tanks”, R. I. Loehrke et al, I. Energy, May-June 1979.
The advantage of using a flexible fabric pipe as a stratification inlet pipe is that the fabric pipe can expand or collapse leading to an equalization of the pressure in the pipe and in the tank. This action tends to prevent inflow and outflow through the porous pipe until a level of equal densities and thus temperatures in the pipe and the tank has been reached. The liquid in the pipe will first enter the tank when it either reaches the top of the pipe, where it is forced to leave the pipe, because new water is constantly fed into the pipe, or when the temperature in the pipe equals the temperature in the tank leading to a slightly higher pressure in the pipe than in the tank. The pipe will expand in an attempt to equalize the pressure difference, but the expansion is limited by the expansion properties of the fabric which leads to a flow of liquid from the pipe into the tank at the right temperature level.
Because a single layer of fabric is very thin, heat is constantly transferred from/to the fabric pipe to/from the tank resulting in impaired stratification.